If you have any "active content" on your webpage that loads from an APPLET, EMBED, or OBJECT HTML element, your page is gonna act funny in the upcoming version of IE.
I don't know if or how this'll affect future versions of Firefox and/or Opera. The patent-owner, Eolas, might just be going after the deepest pockets first.
The upshot is that, to comply with the patent, active content in web pages on IE 7 must be activated. That means when you click on an active webpage element, it'll pop up a tooltip saying that you'll have to click it again to interact with the content as shown here. After that, the applet will work normally.
Easily worked around? Fortunately yes.
If you're using Flash, an easy little hack is here. It works just fine in every browser I tried and adds a few little niceties like player version detection.
If you're running Java applets or Shockwave or something else, google around for a similar hack.
Also fortunately, wrapping your applets this way is reasonably future-proof. If an industry consortium appears and crushes this patent like a bug, you won't have to return to pull all of your workaround code out.
I just did it with all of my stuff, so if you see any of my pages suddenly acting funny, lemme know.